llvm-c: Make LLVMGetFirstTarget a proper prototype

This avoids warnings when included in a application that
uses -Wstrict-prototypes.

Differential Revision: http://llvm-reviews.chandlerc.com/D1713



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191029 91177308-0d34-0410-b5e6-96231b3b80d8